The present invention relates to a honeycomb structure and a manufacturing method thereof. More specifically, the present invention relates to a honeycomb structure made of cordierite having excellent thermal shock resistance even with large pore diameters and high porosity and to a method for manufacturing such a honeycomb structure, the method being capable of manufacturing such a honeycomb structure with high precision in size.
In recent years is high-lightened an influence of harmful substances such as particulate and NOx contained in exhaust gas discharged from automobile engines such as a diesel engine on the environment. As an important means for removing such harmful substances, a use of a honeycomb structure is variously examined.
FIG. 1 is a perspective view schematically showing an example of a honeycomb structure. As shown in FIG. 1, the honeycomb structure 1 has a plurality of through-holes (cells 3) separated from each other by porous partition walls 2 and functioning as fluid passages. The honeycomb structure has a structure of plugging in mutually different sites at both end faces where the cells are open, and development of use of the honeycomb structure has been proceeding as a filter (honeycomb filter) for trapping and removing particulate matters in exhaust gas by allowing exhaust gas to flow into each of the cells 3 opening at one end face to compulsorily pass the exhaust gas through the porous partition walls 2. Also, development of use of the honeycomb structure has been proceeding as a catalyst body carrying a catalyst which decomposes HC and NOx on the partition walls 2 to purify exhaust gas. As a material constituting such a honeycomb structure, there has suitably been used a cordierite having a low thermal expansion coefficient and high thermal shock resistance.
When a honeycomb structure is used as a filter, it is naturally required that it has high trapping efficiency. After a certain period of use, the honeycomb structure is generally subjected to a regeneration treatment where accumulated soot is combusted at high temperature to be burned down. When the regeneration treatment is conducted with high frequency, deterioration of the honeycomb structure is promoted. Therefore, there is a requirement of reducing the number of regeneration treatments with making trapping time longer.
Meanwhile, when a honeycomb structure is used as a catalyst carrier, there has been required in recent years an increase in an amount of catalyst to be loaded on the honeycomb structure in order to further improve exhaust gas purification performance. In order to increase the amount of catalyst to be loaded, it is desirable to rise porosity of the honeycomb structure. The honeycomb structure having larger pore diameters is more desirable. In any of the cases that a honeycomb structure is used as a filter and that a honeycomb structure is used as a catalyst body, the honeycomb structure is disposed near the engine and exposes to thermal shock continuously. Therefore, the honeycomb structure is required to have sufficient thermal shock resistance.
A honeycomb structure made of cordierite can be manufactured by preparing clay by the use of a cordierite-forming raw material containing an alumina source, forming the clay obtained into a honeycomb shape to obtain a honeycomb formed body, and drying and firing the honeycomb formed body (see, e.g., JP-A-2003-40687). There is also disclosed a honeycomb product made of cordierite using boehmite having a BET specific surface area of 20 m2/g as an alumina source for the purpose of enhancing thermal shock resistance by reducing a thermal expansion coefficient (see, e.g., JP-A-2001-524452).
However, a honeycomb product manufactured in a method disclosed in the Patent Document 2 has a small average pore diameter of about 1.5 μm. Therefore, since such a honeycomb product has high strength, it is disadvantageous to catalyst loading, while thermal shock resistance is improved, and a problem of difficulty in loading a sufficient amount of catalyst arises.